This invention relates to a mass spectrometer for measuring a quantity of ions produced by impacting a sample gas with an electron beam to ionize, introducing the produced ions into a magnetic field or electric field, and separating the ions according to their mass numbers to determine their ion numbers. In particular, this invention relates to a technique for preventing a sample gas from being decomposed at an ion source or in a trap to adhere and deposit on electrodes or the like.
A mass spectrometer is an analyzer in which a sample molecule is collided with the electron beam with several tens of electron volts (eV) to ionize, and the produced ions are introduced into a magnetic field or an electric field to separate according to the mass number. Then, a mass spectrum with the mass number in a horizontal axis and the ion quantity in a vertical axis is created to determine the sample molecules.
The mass spectrometer is classified into a magnetic field type and an electric field type based on the mass separating method. FIG. 3(a) shows a principle of the magnetic field type mass spectrometer, and FIG. 3(b) shows a principle of the electric field type mass spectrometer.
In the magnetic field type mass spectrometer, an instrument is maintained under a high vacuum of 10−6 to 10−8 Torr. A sample gas is introduced into an ion source 10 at a constant flow rate, and the sample gas is subjected to impact of the electron beam having energy of the order of 50 to 100 eV to ionize the sample gas. Acceleration electrodes 2 accelerate the ions from the ion source 10 to enter a magnetic field 3. A path of the ions inside the magnetic field 3 is curved according to the Fleming's left hand rule, and then a detector 5 detects the ions after passing through a collector slit 4. Since the curve radius is different depending on the mass number, a mass spectrum can be obtained.
In the case of the electric field type mass spectrometer, a sample gas is ionized at the ion source 10. Accelerating electrodes 2a accelerate the ions to introduce into an electric field created by quadruple electrodes 3a. A direct current voltage and a high frequency voltage, i.e. ±(U+V cos ωt), are applied to four bar-shape electrodes disposed in parallel to each other. When the ions enter the electric field under a specific frequency condition, only the ions with a specific mass number pass through with specific amplitude defined by the x-axis and y-axis. The ions with other mass numbers have amplitude that exponentially increases with time, and eventually collide with the electrodes. Therefore, only the ions with the specific mass number satisfying the electric field condition can pass through and reach a secondary electron multiplier 5a to be detected. By sweeping the electric field to sequentially change the electric field condition, the mass spectrum is obtained.
A method of ionizing the sample in the ion source 10 of the mass spectrometer includes an electron ionization method (EI method) by an electron and a chemical ionization method (CI method) by a reactive gas ion. The electron ionization method has been most widely used. When an electron beam hits a molecule with energy more than necessary to separate an electron from the outmost orbit of the molecule, a molecular ion (a parent ion) without the electron on the outmost orbit is produced in addition to various ions (fragmented ions) with cut off internal bonds. In the electron ionization method, it is possible to conduct analysis from a mass spectrum of the fragmented ions produced by the fragmentation (ion cleavage). As opposed to the electron ionization method, the chemical ionization method uses an ionization method in a milder condition. As the fragmentation is difficult to take place, information regarding a molecular weight can be obtained.
FIG. 4 is a schematic view showing an ion source 10 according to the conventional electron ionization method. A sample introduction pipe 19 is connected to an ionization chamber 20 disposed in a vacuum atmosphere. A gas sample is introduced into the ionization chamber 20 through the pipe. A filament 11 for generating thermoelectron is disposed outside a thermoelectron irradiating opening 11a with an opening on a wall surface of the ionization chamber 20. When a power current is supplied to the filament 11 from a current source 11b, the temperature of the filament 11 is increased to thereby discharge the thermoelectrons.
The thermoelectrons (e− in FIG. 4) are attracted by a potential difference between the filament 11 and the trap electrode 12 to enter the ionization chamber 20, and further accelerated toward the trap electrode 12. When the thermoelectron beam collides against the sample molecule, electrons are kicked out from the sample molecules, so that the molecules become positive ions. The generated ions jump out of the ionization chamber 20 through the ion exit 21. Then, the acceleration electrodes 2 (or 2a) pull and accelerate the ions as shown in FIGS. 3(a) and 3(b) to introduce into the mass spectrometer system. Since the number of electrons trapped in the trap electrode 12 depends on the number of electrons discharged from the filament 11, a controlling portion 11c controls the current source 11b so that an electric current of the thermoelectrons trapped at the trap electrode 12 becomes a specific value. Thus, the quantity of the thermoelectrons at the filament 11 becomes substantially constant, so that a stable ionization can be attained in the ionization chamber 20.
The conventional mass spectrometer is structured as described above. However, an inner surface of the analysis instrument, especially at the ion source having the electrodes for generating the electric field or the ion trap, is exposed to the sample gas molecules. As a result, a specific sample gas is decomposed and deposited on the surface, causing an unexpected result due to an interaction with the ions. For example, a catalytic reaction due to a chemical reaction may take place on the surface, and an analysis result may be distorted. Also, the surface tends to promote the sample molecules to be deposited and increases a temperature.
The catalytic action of the deposited sample material inside the instrument affects the measurement. To prevent the effect, the following approaches have been proposed: a method in which chrome or chromium oxide is coated on a surface of the electrodes of the ion source and the ion trap; a method in which an organic silane reagent is chemically bonded to the surface; a method in which an inert fused silica is coated on the surface with a thickness of 0.02 to 0.1 μm; and a method in which alumina, silicon nitride, a selected semiconductor material or the like is coated on the surface, or these materials are alternatively coated. In the surface treatment of the inert fused silica, alumina, silicon nitride and the like, an inert non-organic, non-metallic material is coated on the electrode with a minimum thickness to prevent pin-holes, therefore taking advantage of insulation and the electric field formation. However, it is not easy for an operator to perform such surface treatments. Therefore, the ion source and ion trap are difficult to be maintained by the operator.
In view of the above problems, the present invention has been made and an object of the invention is to provide a mass spectrometer wherein an analyst can easily carry out maintenance of an ion source and ion trap.
Further objects and advantages of the invention will be apparent from the following description of the invention.